Return a tuple containing the command line, and the mailcap entry
used; (None, None) if no match is found. This may invoke the
'test' command of several matching entries before deciding which
entry to use.

The dictionary maps a MIME type (in all lowercase, e.g. 'text/plain')
to a list of dictionaries corresponding to mailcap entries. The list
collects all the entries for that MIME type from all available mailcap
files. Each dictionary contains key-value pairs for that MIME type,
where the viewing command is stored with the key "view".

Each MIME type is mapped to an entry consisting of a list of
dictionaries; the list will contain more than one such dictionary
if a given MIME type appears more than once in the mailcap file.
Each dictionary contains key-value pairs for that MIME type, where
the viewing command is stored with the key "view".